Anaerobic digestion is a known process used to sterilize solids and biosolids and involves the decomposition of organic and inorganic matter under anaerobic conditions, that is in the absence of molecular oxygen. Major applications of anaerobic digestion are in the stabilization of concentrated sludges produced in the course of municipal and industrial wastewater treatment. Anaerobic digestion processes produce gas that in the case of municipal wastewater sludge, for example, can be used to meet most of the energy needs for plant operation.
In recent years, there has been considerable effort to design pre-treatment systems that increase the efficiency of anaerobic digesters. These pre-treatment processes are typically designed to alter physical and chemical properties of sludge in order to make them more readily degradable during the course of anaerobic digestion. One emerging pre-treatment process that is receiving a great deal of attention is thermal hydrolysis. A thermal hydrolysis process causes cell walls to rupture under conditions of high temperature and high pressure and generally results in highly solubilized sludge which is more easily biodegradable. In particular, thermal hydrolysis employs high temperatures on the range of 130° C. to 180° C. and high pressure, typically in the range of 5-8 bar. By decoupling long chain polymers and hydrolyzing the proteins, thermal hydrolysis transforms the sludge in ways that makes anaerobic digestion more efficient. Further, thermal hydrolysis is able to split and decompose a significant part of the sludge solid fraction into soluble and less complex molecules. It is contemplated that an anaerobic digestion process can achieve 55-60% volatile solids destruction after an appropriate thermal hydrolysis process.
Thermal hydrolysis, while substantially improving the efficiency of anaerobic digestion, has some drawbacks. Sludge discharged from a thermal hydrolysis reactor is typically at a relatively high temperature, for example on the order of 150° C.-165° C. Thus, the thermal hydrolyzed sludge must be cooled before it reaches the anaerobic digestion process that typically operates in the range of 37° C.-42° C. For cooling the thermally hydrolyzed sludge, heat exchangers are typically employed. Because the thermal hydrolyzed sludge is at a temperature substantially over 100° C., it must be maintained under pressure to prevent the sludge from boiling. Thus, the heat exchangers employed to cool the thermally hydrolyzed sludge must be able to handle the thermally hydrolyzed sludge at pressures on the order of 100-120 psi, for example. This greatly complicates the construction of the heat exchangers and substantially increases the initial cost of the heat exchangers. Furthermore, heat exchangers designed to be used in such high pressure applications require frequent cleaning and maintenance and this also leads to high operating costs.
Therefore, there has been and continues to be a need for a wastewater treatment process that includes a cost effective system and process for treating resulting primary and biological sludge.